Automobile differential gear system of the prior art have three functions: ON, OFF and LOCK. The ON and OFF functions are traditionally assigned in a common control unit, whereas the LOCK function is attributed to another mechanism. However, this design does not provide adroit control. The invention disclosed by U.S. Pat. No. 5,997,428 is an example, in which the input shaft for the differential gear is switchable between ON and OFF. But the shaft is designed to be a double-section type, having merely the functions of ON and OFF.
On the other hand, the inventions disclosed by U.S. Pat. Nos. 6,634,978 and 6,450,915 have the functions ON, OFF and LOCK are all disposed behind a side gear on one side of a planetary gear carrier, wherein a slide sleeve will be shifted stepwise to engage and drive the side gear or the planetary gear carrier. However, the engagement between the sleeve and the planetary gear carrier is realized behind the side gear on the inner rim of the outer shell of the sleeve. Therefore, not only the size of the control mechanism is enlarged but also the weight and the production cost are increased.